<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        console.log(Array.prototype);
        Array.prototype.sum = function() {
                var sum = 0;
                for (var i = 0; i < this.length; i++) {
                    sum += this[i];
                }
                return sum;
            }
            // Array.prototype = {
            //     sum: function() {
            //         var sum = 0;
            //         for (var i = 0; i < this.length; i++) {
            //             sum += this[i];
            //         }
            //         return sum;
            //     }
            // }
        var arr = [1, 2, 3, 4, 5];
        console.log(arr.sum());
    </script>
</body>

</html>